A sharp decline in June trading volume pushed CME Group stock to its lowest point since the start of 2006, further slashing the value of it ever-shrinking bid to acquire the New York Mercantile Exchange.

CME Group shares closed down 6.2 percent Tuesday, at $359.39, a little more than half of what the parent company of the Chicago Mercantile Exchange and Chicago Board of Trade was worth in December.

When CME Group announced negotiations in January to buy the Nymex, it offered a combination of cash and stock valued at more than $11 billion, or $119 per share.

The stock slid in the following months amid questions about federal opposition to futures exchanges owning clearinghouses, eroding the value of the deal to an unacceptable level for some Nymex shareholders and members whose votes could decide its fate. The plunge in CME Group shares Tuesday reduced the deal to about $7.85 billion, roughly $3 per share above Nymex's closing price of $80.46...